1. Field of the Invention
The present invention relates to a gene reporter assay and a kit for determining the presence and/or the level in a sample of an extracellular signal that activates the signal transduction activity of a cell surface molecule or complex. The present invention further relates to a cell line which can be used in such an assay.
2. Description of the Related Art
Cell surface proteins permit intracellular transduction of extracellular signals. Cell surface proteins provide eukaryotic, as well as prokaryotic, cells a means to detect extracellular signals and transduce such signals intracellularly in a manner that ultimately results in a cellular response or a concerted tissue or organ response. Cell surface proteins, by intracellularly transmitting information regarding the extracellular environment via specific intracellular pathways induce an appropriate response to a particular stimulus. The response may be immediate and transient, slow and sustained, or some mixture thereof. By virtue of an array of varied membrane surface proteins, eukaryotic cells are exquisitely sensitive to their environment.
Extracellular signal molecules, such as cytokines, growth factors, hormones, vasodilators and neurotransmitters, exert their effects, at least in part, via interaction with cell surface proteins. For example, some extracellular signal molecules cause changes in transcription of target gene via changes in the levels of secondary messengers, such as cAMP. Other signals indirectly alter gene expression by activating the expression of genes, such as immediate-early genes that encode regulatory proteins, which in turn activate expression of other genes that encode transcriptional regulatory proteins. Other extracellular signal molecules cause activation of latent cytoplasmic signal transducers and activators of transcription (STAT) protein that enhance the transcription of specific sets of genes.
Cell surface receptors and ion channels are among the cell surface proteins that respond to extracellular signals and initiate the events that lead to this varied gene expression and response. Ion channels and cell surface-localized receptors are ubiquitous and physiologically important cell surface membrane proteins. They play a central role in regulating intracellular levels of various ions and chemicals, many of which are important for cell viability and function.
Cell Surface Receptors
Cell surface-localized receptors are membrane spanning proteins that bind extracellular signalling molecules or changes in the extracellular environment and transmit the signal via signal transduction pathways to effect a cellular response. Cell surface receptors bind circulating signal molecules, such as cytokines, growth factors and hormones, etc., as the initiating step in the induction of numerous intracellular pathways. Receptors are classified on a structural basis or on the basis of the particular type of pathway that is induced. Among these classes of receptors are classes of cytokine receptors which include those that bind growth factors and have intrinsic tyrosine kinase activity, such as the heparin binding growth factor (HBGF) receptors, the immunoglobulin receptor superfamily, the hematopoietin/cytokine receptor superfamily, the nerve-growth factor receptor superfamily, other receptor tyrosine or serine kinases, and those that couple to effector proteins through guanine nucleotide binding regulatory proteins, which are referred to as G protein coupled receptors and G proteins, respectively.
Cytokines are intercellular messengers which coordinate communication between cells within a particular tissue, for example, antibody and T cell immune system interactions, and serve to modulate or modify the biological response. They are pleiotropic and have a broad spectrum of biological effects on more than one type of cell or tissue. The receptors for cytokines are broadly grouped into two classes, where the Class I cytokine receptors include receptors that bind various interleukins (IL-2, IL-3, IL-4, IL-6, IL-7, IL-9, IL-11, IL-12, IL-15), erythropoietin (EPO), growth hormone (GH), granulocyte colony stimulating factor (G-CSF), granulocyte macrophage colony stimulating factor (GM-CSF), leukemia inhibitory factor (LIF), and ciliary neurotrophic factor (CNTF), and the Class II cytokine receptors include receptors that bind interferon (IFN) α/β, IFNγ, and IL-10.
Interferon Receptors
Human interferons (IFNs) are a family of homologous helical cytokines composed of three distinct classes: type I, type II, and type III based on nucleotide and amino acid sequence homology. Human Type I IFNs consist of IFN-α, IFN-β, IFN-ε, IFN-κ, and IFN-ω. Human IFN-α includes a group of closely related proteins encoded by at least 12 functional IFN-α genes. IFN-β, IFN-ε, IFN-κ, and IFN-ω, are encoded by single more distantly related genes. Type II IFN, or IFNγ, is encoded by an unrelated gene and binds to a distinct cell surface receptor (De Maeyer et al., 1988; Pestka et al., 1987 and Diaz et al., 1993). Recently, a novel group of interferons designated IFN-λ or type III IFNs has been described. The group has three members IFN-λ1, IFN-λ2, and IFN-λ3 also termed interleukin-29 (IL-29) (λ1), and IL-28A/B (λ2/3). (Sheppard et al., 2003; and Ank et al., 2006).
Type I IFNs bind to a common receptor, as shown by their ability to cross-compete for receptor binding (Pestka et al., 1987; Branca et al., 1981; and Merlin et al., 1985). The Type 1 interferon receptor has the largest number of natural ligands, some 14 in all, of all known cytokine receptors. Binding of interferons to their cell surface receptor represents the initial and probably most specific step in the IFN signaling pathway.
The Type I IFN receptor is composed of two transmembrane glycoproteins, IFNAR1 and IFNAR2 (Uze et al., 1990; Novick et al., 1994; Lutfalla et al., 1995; Domanski et al., 1995), which are rapidly tyrosine-phosphorylated following IFN binding (Platanias et al., 1994; Constantinescu et al., 1994; and Abramovich et al., 1994). Both subunits belong to the class II cytokine receptor superfamily (Bazan et al., 1990 and Thoreau et al., 1990) and are required for high affinity ligand binding and the establishment of biological activity (Langer et al., 1996 and Domanski et al., 1996). Class II cytokine receptors are distinguished from Class I receptors on the basis of the pattern of the conserved pairs of cysteine residues that are thought to form disulfide bonds.
The Type II IFN (IFN γ) receptor is composed of two transmembrane glycoproteins, IFNGR1 and IFNGR2 that are preassembled at the cell surface. Binding of IFN γ to its receptor activates the tyrosine kinases Jak1 and Jak2 resulting in tyrosine-phosphorylation and formation of a Stat1 homodimer. The activated Stat1 homodimer is then translocated to the nucleus where it binds to the GAS (Gamma Activated Sequence) resulting in transcriptional activation of IFN γ activated genes.
Type III interferons bind to a unique receptor comprising the IL-28Rα, which is specific for a chain of the IFN-λs, and the IL-10Rβ chain which is also part of the receptors for IL-10, IL-22, and IL-26 (Ank et al, 2006).
In contrast to other cytokine receptors, particularly the IFN-γ receptor, neither IFNAR1 nor IFNAR2 alone bind to IFNα or IFNβ with an affinity comparable to the heterodimer. Despite the fact that IFNAR2 plays a prominent role in ligand binding, IFNAR1 contributes to IFN binding by increasing the affinity of the receptor complex (4-10 fold) relative to that of IFNAR2 alone. IFNAR1 also modulates the specificity of ligand binding relative to that observed with IFNAR2 alone (Cohen et al., 1995; Russell-Harde et al., 1995; Cutrone et al., 1997; and Cook et al., 1996). IFNAR1 has a larger extracellular domain than most other class II cytokine receptors, composed of 4 immunoglobulin-like subdomains separated by di- or tri-proline motifs which can be divided into two tandem repeats (Novick et al., 1994; Lutfalla et al., 1992; and Uzé et al., 1995).
Human, murine and bovine IFNAR1 have been cloned and expressed in human and murine cells. Studies performed with transfected cells show that IFNAR1 plays a central role in ligand binding, cellular responses to IFNs and in the induction of the biological activities of the Type I interferons (Novick et al., 1994; Abramovich et al., 1994; Uzé et al., 1992; Mouchel-Vielh et al., 1992; Lim et al., 1993; Cleary et al., 1994; Constantinescu et al., 1995; Hwang et al., 1995; Vandenbroek et al., 1995; and Colamonici et al., 1994). The IFN receptor also determines the high degree of species specificity characteristic of the IFNs. Thus, transfection of mouse cells with IFNAR1 and IFNAR2 renders mouse cells sensitive to human type I IFNs since both human and mouse cells share a common signaling pathway and common IFN responsive elements in the promoter regions of IFN regulated genes. Furthermore, the intracellular domain of IFNAR1 has been shown to play a key role in the transduction of the signal initiated at the cell surface by binding of Type I interferons to the nucleus (Basu et al., 1998). Targeted disruption of the IFNAR1 gene results in the loss of the antiviral response to Type I IFNs demonstrating that this receptor polypeptide is an essential component of the receptor complex and that both IFNAR1 and IFNAR2 subunits are required for IFNα and IFNβ signaling (Vandenbroek et al., 1995; Muller et al., 1994; Fiette et al., 1995; Steinhoff et al., 1995; and van den Broek et al., 1995).
Binding of type I interferon to the receptor complex activates two Janus kinases, Tyk2 and JAK1, which mediate the tyrosine phosphorylation and activation of two latent cytoplasmic transcription factors STAT1 and STAT2 which form a complex (ISGF3) with a p48 DNA binding protein, interferon responsive protein 9 (IRF 9), which is translocated to the nucleus to promote specific gene transcription (Fu et al., 1992; Schindler et al., 1992; Darnell et al., 1994; Ihle et al, 1995; and Taniguchi, 1995). Both Tyk2 and STAT2 are constitutively associated with the membrane proximal region of the IFNAR1 chain, while JAK1 and STAT1 are physically associated with IFNAR2 and all four factors are rapidly activated during IFNα stimulation (Lutfalla et al., 1995; Bazan, 1990; Basu et al., 1998; Barbieri et al., 1994; Velazquez et al., 1995; Uddin et al., 1995; Yan et al., 1996(a) and 1996(b).
Binding of type III IFNs to their cell-surface receptor also activates the ISGF3 complex suggesting that type III IFNs also activate a number of genes in common with type I IFNs (Ank et al., 2006).
Key populations of cells including DCs distributed throughout the peripheral tissues act as sentinels capable of recognizing infectious agents through pattern-recognition receptors (PRR). These include the Toll-like receptor (TLR) family of cell surface and endosomal membrane receptors (Uematsu and Akira, 2007) and the retinoic acid-inducible gene I (RIG-I)-like cytosoloic receptor proteins RIG-I, MDA5, and LGP2 (Yoneyama and Fujita, 2007). Thirteen members of the TLR family have been identified in mammals (Uematsu and Akira, 2007). Each TLR mediates a distinctive response in association with different combinations of four Toll/IL-1 receptor (TIR) domain-containing adaptor proteins (MyD88, TRIP, TIRAP/MAL, and TRAM). All the TLRs except TLR3 interact with MyD88. TLR3, which recognizes single-stranded or double-stranded viral RNA, is localized in the endosomes of myeloid DCs and requires acidification of vesicles for activation. TLR3 signals via TRIF and activates TBK1/IKKe which phosphorylates the interferon regulatory factor 3 (IRF3) and NFkB, resulting in production of IFN b (Hemmi et al, 2004, Perry et al., 2004). The RIG-1-like receptor proteins are DExD/H box RNA helicases two of which, RIG-I and MDA5, carry caspase activation and recruitment domain (CARD)-like motifs at the N-terminus (Yoneyama and Fujita, 2007). The CARD domain interacts with IPS-1 resulting in activation of IRF3 and NFkB and production of IFN b. Thus, activation of PRRs leads to the production of pro-inflammatory cytokines including type I IFNs and activation of the innate immune response. Dendritic cells signal principally through TLRs while RIG-1-like receptors predominate in other cell types. Two major DC sub-sets can be distinguished in man, CD11c(+) monocyte derived myeloid DCs, present in most tissues, and CD11c(−) plasmacytoid DCs (pDCs), present principally in lymph nodes. Plasmacytoid DCs are the principal producers of type I IFNs in response to viruses (Steinmann and Hemmi, 2006). Plasmacytoid DCs express high levels of TLR7/8 and TLR9 that recognize single-stranded RNA (ssRNA) and CpG DNA respectively (Diebold et al., 2004, Heli et al., 2004). Hemmi et al., 2000). Activation of both TLR7/8 and TLR9 leads to the formation of a complex with MyD88 and phosphorylation of IRF7 and production of high levels of type I IFNs (Uematsu and Akira, 2007).
TNF Receptors
Tumor necrosis factor alpha (TNF-α) is a multifunctional cytokine that exerts pleiotropic effects on different cell types. TNF-α is synthesized as pro-TNF, a 26 kDa membrane bound protein, which is released upon cleavage of its pro domain by TNF-converting enzyme (TACE) to yield a 17 kDa protein consisting of 157 amino acids that exists as a homotrimer in solution. TNF-α bind to two distinct receptors TNFR-1 (p55) and TNFR2 (p75). TNFR1 contains a death domain (absent from TNFR2) which is involved in the induction of apoptosis. Binding of the TNF-α homotrimer to TNFR-1 results in trimerization of TNFR-1 and the silencer of death domain (SODD) is released. The TNFR-associated death domain (TRADD) binds to the death domain of TNFR-1 and recruits the adaptor proteins, receptor interacting protein (RIP), TNFR-associated factor 2 (TRAF-2), and the Fas-associated death domain (FADD). TNFR-1 signals apoptosis, by FADD binding pro-caspase-8 the activation of which leads to induction of a protease cascade resulting in apoptosis. TNFR-1 signals survival by recruitment of TRAF-2 which inhibits apoptosis via the cytoplasmic inhibitor of apoptosis protein (cIAP). One of the principal signaling pathways triggered by recruitment of TRAF-2 and RIP to the TNFR-1 receptor complex is the NF-κB pathway which transduces a signal to the nucleus culminating in transcriptional activation of a number of TNF target genes (Schwamborn et al., 2003). NF-κB is a ubiquitous transcription factor induced by a number of cytokines (including IFNγ, IL2, IL5 and IFNα2). NF-κB is involved in the regulation of numerous genes involved in processes including, the inflammatory response, apoptosis, cancer, neuronal survival, and innate immunity. Activation of NF-κB is controlled principally at the posttranscriptional level by degradation of the inhibitory subunit IκB of the p55/p65/IκB complex present in the cytoplasm. Activating stimuli such as TNFα activate a kinase complex composed of two IκB-specific kinases (IKKα and IKKβ) and a modulatory subunit (NEMO or IKKγ). This leads to phosphorylation of the inhibitory subunit, which is then ubiquitinylated and degraded via the proteasome. This triggers translocation of NF-κB into the nucleus, where it initiates transcription by binding to regulatory sequences (NF-κB recognition/binding sequences) present in the promoter region of NF-κB target genes.
G-Coupled Receptors
The G protein transmembrane signaling pathways consist of three proteins: receptors, G proteins and effectors. G proteins, which are the intermediaries in transmembrane signaling pathways, are heterodimers and consist of α, β and γ subunits. Among the members of a family of G proteins the α subunits differ. Functions of G proteins are regulated by the cyclic association of GTP with the α subunit followed by hydrolysis of GTP to GDP and dissociation of GDP.
G protein coupled receptors are a diverse class of receptors that mediate signal transduction by binding to G proteins. Signal transduction is initiated via ligand binding to the cell membrane receptor, which stimulates binding of the receptor to the G protein. The receptor G protein interaction releases GDP, which is specifically bound to the G protein, and permits the binding of GTP, which activates the G protein. Activated G protein dissociates from the receptor and activates the effector protein, which regulates the intracellular levels of specific second messengers. Examples of such effector proteins include adenyl cyclase, guanyl cyclase, phospholipase C, and others.
Growth Factors and Growth Factor Receptors
Polypeptide growth factors are modulators of cell proliferation and differentiation whose biological functions are mediated by the interaction of the growth factor with cell surface receptors and subsequent alterations in gene expression. Growth factors bind to specific receptors and appear to induce tyrosine phosphorylation and c-fos mRNA synthesis. In addition, at least some growth factors, such as platelet-derived growth factor (Yeh et al., 1987) and heparin-binding growth factor-2 or basic fibroblast growth factor (Bouche et al., 1987), are translocated to the nucleus.
Activation of growth factor receptors by interaction with specific growth factors or with agents such as phorbol mistric acetate (PMA) activates protein kinase C, which is a family of phospholipid- and calcium-activated protein kinases. This activation results in the transcription of an array of proto-oncogene transcription factor encoding genes, including c-fos, c-myc and c-jun, proteases, protease inhibitors, including collagenase type I and plasminogen activator inhibitor, and adhesion molecules, including intercellular adhesion molecule I. Protein kinase C activation antagonizes growth factor activity by the rapid phosphorylation of growth factor receptors, which thereby decreases tyrosine kinase activity. Growth factors and other mitogens that induce cell proliferation and cell growth are believed to play a role in tumor growth, which often carry identifiable cell surface receptors specific for growth factors and other extracellular signals.
The interaction of nerve growth factor (NGF) with its receptor is typical of the array of responses such an extracellular signal induces. NGF is a polypeptide growth hormone that is necessary for differentiation and growth of the neural crest-derived sensory neuron. NGF binds to its specific cell surface receptor and is retrogradely transported to the cell body (Changelian et al., 1989). This initiates a cascade of intracellular events, culminating in a differentiated phenotype. PC12 cells, which are a rat pheochromocytoma cell line, are used as a model for the study of NGF-mediated differentiation. When treated with NGF, PC12 cells change from replicating adrenal-chromaffin-like cells to nonreplicating, electrically excitable sympathetic-neuron-like cells.
Concomitant with the phenotypic changes, there is induction and expression of specific genes. Binding of NGF to PC12 cells induces the immediate and rapid expression of certain genes, including the c-fos, NGF1-A and NGF1-B genes, which are referred to as early genes. Such early genes are believed to encode transcriptional regulators. The NGF-1A gene product contains tandemly repeated “zinc finger” domains that are characteristic of DNA-binding proteins, and the NGF1-B protein is homologous to members of the glucocorticoid receptor family and, thus, may function as a ligand-dependent modulator of transcription. The c-fos gene product, FOS appears to function as a transcriptional regulatory molecule.
The c-fos Gene and Related Genes
As discussed above, induction of expression of the c-fos gene is an event that is common to a number of response pathways that are initiated by the activity of a variety of cell surface proteins.
The c-fos gene product, FOS, associates with the transcription activator JUN, which is the product of the c-jun gene, to form a complex that forms a transcription activation complex, AP-1. Transcription of both c-fos and c-jun is induced rapidly and transiently following stimulation. The induced mRNAs accumulate for 1-2 hours in the cytoplasm where the FOS and JUN proteins, which are short-lived, are translated and then translocated to the nucleus to form a heterodimeric protein complex that binds to the DNA regulatory element, AP-1 binding site.
The c-fos and c-jun genes are members of gene families that encode proteins that participate in the formation of heterodimeric complexes that interact with AP-1 binding sites. Transcription factor AP-1 is composed of several protein complexes whose concentrations change upon cell stimulation. These complexes specifically interact with a seven-base core nucleotide sequence motif, that is known to be a relatively common constituent of both positive and negative transcriptional regulatory elements and that is required for both basal and induced levels of gene expression.
The gene products, FOS and JUN cooperate in the regulation of target genes that underlie many cellular and adaptive responses to the environment. They are involved in a number of neurophysiological processes.
Thus, c-fos induction involves distinct second messenger pathways that act via separate regulatory elements and that differentially modify, the resulting gene product, FOS, which in turn interacts in different ways with differentially modified JUN protein. Therefore, a multitude of extracellular events induce expression of a small number of inducible proteins that form an array of protein complexes that can differentially bind to DNA regulatory elements that contain AP-1 binding sites. Therefore, numerous cell surface proteins can act via overlapping transduction pathways and transduce extracellular signals that ultimately induce a variety of responses.
There are many assays that may rely on in vivo activity in a living cell line. One example is a cell line having an Interferon Stimulatory Response Element (ISRE) connected to a luciferase gene, or another reporter gene, so that when the cell line is subjected to the presence of interferon as an extracellular signal, the signal transduction activity of endogenous interferon cell surface receptors produces a signal that activates the ISRE, which then causes transcription of the luciferase gene. Thus, the activity of luciferase in creating light can be measured and is related to the amount of interferon which is present in the sample, and which is proportional to the amount of interferon over a particular range (Lallemand et al., 1996).
Lleonart et al. (1990) described a reporter gene assay for Type I interferon based on monkey Vero cells transfected with Type I interferon inducible mouse Mx promoter linked to the human growth hormone (hGH) gene as the reporter gene. This Type I interferon assay was developed further by transfecting monkey Vero cells with a plasmid carrying the luciferase reporter gene under the control of the Type I interferon inducible mouse Mx1 promoter (Canosi et al., 1996).
A further type of interferon reporter gene assay was developed by Hammerling et al. (1998) who used a human glioblastoma cell line transfected with a reporter gene construct of glial fibrillary acidic protein (GFAP) promoter and an E. coli β-galactosidase (lacZ) reporter gene. In this particular assay, it is the reduction/inhibition of β-galactosidase expression by either human Type I or Type II interferon in a selective and dose dependent manner that is measured.
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